The present invention relates to a non-invasive and simplified method and apparatus for monitoring the cardiovascular condition of an individual. The invention is particularly useful for monitoring the degree of arteriosclerosis and/or arterial obstruction in an individual, and is therefore described below with respect to this application.
Arteriosclerosis is a condition in which arteries become thick and hard and loose their supple and elastic quality primarily as a result of aging and the deposit of fats, etc. on the vessel walls. It accounts for a large proportion of heart attacks and ischemic conditions produced by an inadequate blood supply to a region caused by a constriction or obstruction of a blood vessel in the arterial vascular system. It also accounts for many strokes, numerous instances of peripheral vascular disease, and most aneurysms of the aorta, which can rupture and cause fatal hemorrhage.
The loss of elasticity of, and the deposit of fats on the artery walls are believed to be part of the normal aging process. However, arteriosclerosis is more likely to occur, or to increase in severity, in individuals who are overweight, smoke, have high blood pressure, suffer from diabetes, or have a family history of high cholesterol. Many techniques have been devised, both non-invasive and invasive, for monitoring the cardiovascular condition of an individual, and particularly for detecting the degree of arteriosclerosis or arterial obstruction that may be present.
An object of the present invention is to provide a novel non-invasive and simplified method and apparatus for monitoring the cardiovascular condition of an individual, which method and apparatus can be implemented in a relatively simple and non-invasive manner. Another object of the invention is to provide a method and apparatus for detecting, and measuring the degree of, arteriosclerosis or arterial obstruction in an individual in a simple and non-invasive manner.
According to one aspect of the present invention, there is provided a method of monitoring the cardiovascular condition of an individual, comprising: detecting an ECG signal of the individual""s heart; detecting a blood front wave in a peripheral artery of the individual; and measuring the time lag between a predetermined reference point in the detected blood front wave and a predetermined reference point in the ECG signal such as to provide an indication of the presence of arteriosclerosis in the individual.
The peak of the R-wave in the ECG signal, and the start of the blood front wave, have been found to be particularly good predetermined reference points since both are generally sharply defined.
According to a further feature in the described preferred embodiments of the invention, the blood front wave is detected non-invasively by applying a probe to a body extremity, such as a finger or toe, supplied by the peripheral artery.
This aspect of the invention thus stems from the discovery of the correlation between the cardiovascular condition of an individual and the time lag experienced by the blood front wave in a peripheral artery of the individual from the ECG signal accompanying the individual""s heart action. It has been found that in healthy individuals this time lag varies approximately linearly with age, decreasing about 1 ms for each year. Thus, when a finger probe is used in healthy individuals of about 14-22 in age, this time lag was found to be about 220 ms; and in individuals of about 50-55 in age, it was found to be about 180 ms. On the other hand, in individuals in the age group of 50-55 suffering from diabetes and other conditions affecting the cardiovascular system, this time lag was found to be about 140 ms, significantly lower than what would have been expected.
It is believed that this time lag is directly related to the elasticity and degree of obstruction of the walls of the arterial vascular system. As noted above, such elasticity decreases with age because of natural aging and the deposit of plaques, fats, etc. It is believed that walls of high elasticity tend to cushion or damper the blood front wave produced by the heart so as to produce a relatively large time lag from the peak of the detected R-wave to the start of the blood front wave. Accordingly, as the arterial wall is made less elastic by aging and the build-up of deposits, this xe2x80x9ccushioningxe2x80x9d effect is reduced, thereby reducing this time lag. By thus measuring this time lag, it is believed one can provide a fairly accurate indication of the elasticity of the arterial wall, and thereby the degree of deposit build-up on the individual""s arterial walls. If the measured time lag for the particular age of the individual is significantly less than that which would be expected in a healthy individual of the respective age and similar physical build (as indicated above), this would indicate not only the presence of arteriosclerosis, but also the extent of arteriosclerosis in the respective individual.
According to further features in the preferred embodiment of the invention described below, the blood front wave is detected in a body extremity of the individual, particularly in the individual""s finger or toe. This makes the test very simple and non-invasive.
According to a still further feature, the blood front wave is detected by an optical oximeter measuring the oxygen saturation of the blood. Such a detector has been found to output an electrical signal closely following the actual blood front wave produced by the heart action accompanied by the ECG signal. The two signals can be displayed on an oscilloscope, to enable an accurate determination to be made of the time lag between the two signals.
According to another aspect of the present invention, there is provided a method of detecting arteriosclerosis in an individual comprising: detecting the blood front wave in a peripheral artery of the individual; examining the shape of the blood front wave; and comparing its shape with a reference corresponding to the shape of the blood front wave in a healthy person to provide an indication of the presence of arteriosclerosis in the individual.
This aspect of the invention stems from the discovery of the correlation between the shape of the blood front wave in a healthy individual with respect to one having arteriosclerosis or obstruction of an artery. Thus, the blood front wave of a healthy individual includes a hump or step arising by the operation of the mitral valve; whereas an individual suffering from arteriosclerosis does not have such a hump or step. The presence of such a hump or step can be easily discerned in a display of the blood front wave on an oscilloscope, or can be easily detected by a spectral density analyzer which determines the ratio of the high-frequency power component (SHF) to the low-frequency power component (SLF). Thus, in a healthy individual SHF/SLF is generally from 0.05-0.1, so that when this ratio is less than 0.05, this would indicate an arteriosclerosis condition, which would be considered quite severe if there is no hump or step at all in the blood front wave, such that SHF/SLF was equal to zero.
According to a still further aspect of the present invention, there is provided a method of monitoring the cardiovascular condition of an individual, comprising: detecting a first blood front wave in a body part of the individual supplied by a peripheral artery; detecting a second blood front wave in another body part of the individual supplied by a different peripheral artery; and measuring the time lag between predetermined reference points of the first and second blood front waves to provide an indication of the cardiovascular condition of the individual.
For example, in a healthy person, the blood front wave in a toe of the individual lags the blood front wave in a finger of the individual by about 50 ms, increasing a small fraction of a single ms each year of age because of the longer arterial path of a toe as compared to a finger. If this delay is found in an examined individual to be substantially greater than that for a healthy individual of the same age, this would indicate an arteriosclerosis condition particularly in the leg.
This aspect of the invention can thus be used generally to indicate the presence of arteriosclerosis without the need of ECG equipment for detecting the individual""s ECG signal. However, more accurate results are obtainable when the ECG signal is also detected and used for determining the lag in the two blood front waves both with respect to the ECG signal and with respect to each other.
According to still further aspects of the present invention, there is provided apparatus for monitoring the cardiovascular condition of an individual in accordance with the above methods.
As described more particularly below, the method and apparatus of the present invention have been found to be particularly useful for monitoring the elasticity of the individual""s arterial blood vessel, and thereby the degree of arteriosclerosis that may be present in the individual""s arterial vascular system.